The present invention relates to a dispersion compensator and a method of manufacturing the dispersion compensator, in which a dispersion compensating fiber is placed in the form of a coil or bundle within a housing and a space around the dispersion compensating fiber is filled with a filling material.
A dispersion compensator for canceling chromatic dispersion at a 1.55 xcexcm band has been developed to perform long-distance, large-capacity optical transmission at a 1.55 xcexcm band using a single-mode optical fiber that has a zero-dispersion wavelength at a 1.3 xcexcm band. International Publication WO00/41011 discloses a dispersion compensator in which a dispersion compensating fiber having negative chromatic dispersion at a 1.55 xcexcm band is wound to form an optical fiber coil, and an outer surface of the optical fiber coil is surrounded by a resin for maintaining the shape of optical fiber coil.
FIG. 6 is a cross-sectional view showing one example of a dispersion compensator disclosed in International Laid-Open Publication WO 00/41011. Numeral 32 denotes an optical fiber coil, 80 denotes a container, 82 denotes a container lid, and 84 denotes a filling material. The above-cited Publication states that the filling material 84 can be formed of, e.g., a thermosetting or ultraviolet-curing silicone resin having the Young""s modulus of not more than 0.05 kg/mm2 or a highly-viscous jelly-like admixture, which is obtained by swelling butadiene, silicone or other similar rubber with a solvent, such as silicone or naphthene, and adding another resin, etc. as required.
Japanese Patent Application Publication No. 2001-154032 discloses a dispersion compensator employing a filling material in which a platinum catalyst is added. The above-cited Publication states that an isocyanate compound contained in a secondary coating layer of an optical fiber impedes curing of the filling material in the portion which is in contact with the optical fiber, thereby causing the filling material in that portion to be in the liquid or semi-liquid phase, while the filling material in other portion is cured to serve as an elastic solid member.
The present invention is concerned with a method of manufacturing a dispersion compensator, the method comprising the steps of placing a dispersion compensating fiber in the form of a coil or bundle within a housing; filling a space around the dispersion compensating fiber within the housing with a filling material having a viscosity of 0.01 Paxc2x7s to 0.6 Paxc2x7s at normal temperature before curing; and then curing the filling material.
By setting the viscosity of the filling material before curing to be in the above-mentioned range, the coil or bundle of the dispersion compensating fiber is entirely and evenly held with the filling material and brought into an immobile condition. Accordingly, since a movement of the fiber due to its own weight is prevented, the micro-bending and external compression between the winding layers of the fiber and between the fiber and the filling material are hardly induced. As a result, an increase of transmission loss can be suppressed and a change of transmission loss caused by a temperature variation due to heat cycles of the dispersion compensator can be reduced. Those advantages of the present invention are valuable because the dispersion compensating fiber is vulnerable to bending and external compression force.
By placing the dispersion compensating fiber in the form of a loosely wound bundle, the filling material, having a viscosity of 0.01 Paxc2x7s to 0.6 Paxc2x7s at the normal temperature before curing, is permeated into the bundle of the dispersion compensating fiber in a manner such that the filling material fills not only the space around the bundle, but also gaps between individual windings of the dispersion compensating fiber. Therefore, the individual windings of the dispersion compensating fiber can be covered with the filling material. Even though some bubbles are left in the gaps between the individual windings of the dispersion compensating fiber, those bubbles will not cause a fatal effect upon the transmission loss. The above-mentioned bundle of the dispersion compensating fiber can be formed by winding the dispersion compensating fiber over a bobbin barrel, withdrawing the bobbin barrel to obtain massive windings of the dispersion compensating fiber in the form of a coil, and deforming the fiber coil into a loosely wound state. As an alternative, the bundle of the dispersion compensating fiber may be formed by dropping the dispersion compensating fiber directly into the housing while a position at which the dispersion compensating fiber drops in the housing is moved so as to draw a circle relative to an upper surface of the housing.
When filling the space around the dispersion compensating fiber in the housing with the filling material, it is possible to fill the gaps between the individual windings of the dispersion compensating fiber with the filling material, and hence to reduce the amount of remaining bubbles, by applying ultrasonic vibration, mechanical vibration or the like to the housing in which the dispersion compensating fiber is placed and the filling material is filled. Therefore, a change of transmission loss caused by a temperature variation due to heat cycles of the dispersion compensator can be further reduced.
By employing the filling material which has a viscosity of the above-mentioned range before curing and in which a platinum catalyst of not less than 100 ppm and a curing restrainer not more than 100 ppm are added, it is made possible to allow the curing restrainer, which hinders curing for several minutes or several hours, to cause the filling material to permeate into the coil or bundle of the dispersion compensating fiber so as to more completely fill the gaps between the individual windings of the dispersion compensating fiber, and moreover to allow the platinum catalyst to cure the filling material that has been filled in the gaps between the individual windings of the dispersion compensating fiber.
By employing the filling material whose viscosity is in the above-mentioned range before curing and whose contact angle measured 30 seconds after the dripping thereof is larger than 0 degree and not more than 12 degrees with respect to a sheet made of the same coating material as that used for the outermost layer of the dispersion compensating fiber, it is ensured that the filling material permeates into the coil or bundle of the dispersion compensating fiber so as to reach a space around each winding of the dispersion compensating fiber and to fill the gaps between the individual windings of the dispersion compensating fiber. Then the transmission loss of the dispersion compensating fiber on the long-wavelength side can be suppressed. One method for adjusting the contact angle of the filling material within the above-mentioned range is to add a surfactant into the filling material. When the contact angle of the filling material after 30 seconds from its dripping is a relatively large value within the above-mentioned range, some bubbles may remain in the space around the dispersion compensating fiber, but those bubbles will not fatally affect the transmission loss of the dispersion compensating fiber. From the viewpoint of keeping the transmission loss as small as possible, the contact angle of the filling material after 30 seconds from the dripping thereof is preferably not more than 8 degrees.
By employing the filling material that has a viscosity in the above-mentioned range before curing and has a cone penetration of not less than 5 and not more than 200 in the range of not lower than xe2x88x9220xc2x0 C. and not higher than 70xc2x0 C. when a xc2xc cone is used in conformity with the standard of JIS K 2220, the coil or bundle of the dispersion compensating fiber can be held stably for a long term, and excessive stresses on the optical fiber are avoided even when the fiber is subjected to a temperature variation caused by changes in the environment. Accordingly, the small transmission loss of the dispersion compensating fiber on the long-wavelength side can be stably maintained for a long term. If the cone penetration of the filling material is smaller than the above-mentioned range, the loss would be increased at low temperatures during heat cycles. If the cone penetration of the filling material is larger than the above-mentioned range, holding of the coil or bundle of the dispersion compensating fiber would be insufficient, thus resulting in a concern that the loss would increase upon impacts applied. Preferably, the filling material has a cone penetration of not less than 5 and not more than 200 at 25xc2x0 C. when a xc2xc cone is used.
Further, the present invention is concerned with a dispersion compensator, the dispersion compensator comprising a dispersion compensating fiber in the form of a coil or bundle, a housing, and a filling material, wherein the dispersion compensating fiber is placed in the housing, and a space around the dispersion compensating fiber is filled with the filling material, the filling material having a viscosity of 0.01 Paxc2x7s to 0.6 Paxc2x7s at normal temperature before curing.
The filling material used in the above dispersion compensator preferably contains a platinum catalyst of not less than 100 ppm and a curing restrainer of not more than 100 ppm.
In the above dispersion compensator, preferably, the filling material has a contact angle of larger than 0 degrees and not larger than 12 degrees after 30 seconds from the dripping thereof with respect to a sheet made of the same coating material as that used for an outermost layer of the dispersion compensating fiber. A surfactant may be added in the filling material.
In the above dispersion compensator, preferably, the filling material has a cone penetration of not less than 5 and not more than 200 at a temperature in the range of not lower than xe2x88x9220xc2x0 C. and not higher than 70xc2x0 C. when a xc2xc cone is used according to the standard of JIS K 2220. More preferably, the filling material has a cone penetration of not less than 5 and not more than 200 at 25xc2x0 C. when a xc2xc cone is used.